The present invention is directed toward a hydrocarbon conversion catalyst which is used especially for effecting the dehydrocyclization of aliphatic hydrocarbons to aromatics. More particularly, the catalyst enables the conversion of C.sub.6 -plus paraffins to their corresponding aromatics with a high degree of selectivity thereby enabling the facile production of large quantities of aromatics.
In the past, it has become the practice to effect conversion of aliphatic hydrocarbons to aromatics by means of the well-known catalytic reforming process. In catalytic reforming, a hydrocarbonaceous feedstock, typically a petroleum naphtha fraction, is contacted with a Group VIII-containing catalytic composite to produce a product reformate of increased aromatics content. The naphtha fraction is typically a full boiling range fraction having an initial boiling point of from about 10.degree.-38.degree. C. and an end boiling point of from about 107.degree.-218.degree. C. Such a full boiling range naphtha contains significant amounts of C.sub.6 -plus paraffinic hydrocarbons and C.sub.6 -plus naphthenic hydrocarbons. As is well known, these paraffinic and naphthenic hydrocarbons are converted to aromatics by means of multifarious reaction mechanisms. These mechanisms include dehydrogenation, dehydrocyclization, and isomerization followed by dehydrogenation. Accordingly, naphthenic hydrocarbons are converted to aromatics by dehydrogenation. Paraffinic hydrocarbons may be converted to the desired aromatics by dehydrocyclization and may also undergo isomerization. Accordingly then, it is apparent that the number of reactions taking place in a catalytic reforming zone are numerous and the typical reforming catalyst must be capable of effecting numerous reactions to be considered usable in a commercially feasible reaction system.
Because of the complexity and number of reaction mechanisms ongoing in catalytic reforming, it has become a recent practice to develop highly specific catalysts tailored to convert only specific reaction species to aromatics. Such catalysts offer advantages over the typical reforming catalyst which must be capable of taking part in numerous reaction mechanisms. Ongoing work has been directed toward producing a catalyst for the conversion of paraffinic hydrocarbons, particularly having six carbon atoms or more, to the corresponding aromatic hydrocarbon. Such a catalyst can be expected to be much more specific resulting in less undesirable side reactions such as hydrocracking. As can be appreciated by those of ordinary skill in the art, increased production of aromatics is desirable. The increased aromatic content of gasolines, a result of lead phase down, as well as demands in the petrochemical industry make C.sub.6 -C.sub.8 aromatics highly desirable products. Accordingly, it would be most advantageous to have a process and a catalytic composition which is highly selective for the conversion of less valuable C.sub.6 -plus paraffins to the more valuable C.sub.6 -plus aromatics.
To formulate catalysts capable of effecting the required reactions, it has been increasingly popular to employ crystalline aluminosilicate zeolites in combination with catalytically active metals. A well known method of preparing catalysts containing zeolites is to incorporate the zeolites into refractory inorganic matrices. Primarily, the use of such matrices, sometimes referred to as binders, has typically been directed towards simplification of catalyst manufacture, providing a simple solution to the problem associated with handling the catalytically active microparticles of zeolite. The microparticles of zeolite are combined with the binder to form or shape macroparticles which are then easily handled and utilized, for example, in a chemical reactor. Before or after forming the zeolite/binder composite, various catalytically active metals can be incorporated into the composite depending on the particular reaction to be catalyzed. Although the zeolite and the metals supply the primary catalytic effect, the contribution to the overall catalytic reaction from the binder and the particular method used to form the composite cannot be ignored. Simple changes in formulation, such as, changing from 100% alumina as the binder material to a mixture of alumina and silica can have a dramatic effect on catalytic performance. Likewise, the use of either acidic or basic solutions during preparation of the catalyst can have an effect on the catalytic performance of the finished catalyst. Therefore, with this in mind, broad general teachings relating to catalyst preparation do not typically lead one skilled in the art to design an effective catalyst formulation for specific applications, such as, the reforming of aliphatic hydrocarbons to aromatics.
Of the body of art that relates to the preparation of catalysts containing zeolites, U.S. Pat. No. 4,507,396 does mention that various zeolites, including L-zeolite, can be formed with colloidal inorganic oxide materials, such as, formed silica, to produce solid inorganic bodies. The preparation method disclosed in this reference requires that the zeolite and the colloidal oxide be dispersed in a water-immiscible solvent and then titrated with an aqueous phase to produce a hydrous plastic agglomerate. U.S. Pat. No. 4,434,311 also teaches that L-zeolite can be combined with an inorganic oxide to prepare catalyst particles. In particular, it is mentioned that the L-zeolite can be mixed with a colloidal suspension of silica in water, stabilized with a small amount of alkali, and extruded to form cylindrical pellets. Extrusion aids selected from ethylene glycol and stearic acid may also be employed.
Another reference, U.S. Pat. No. 4,582,815, is believed most germane to the invention disclosed herein. This reference is directed at a preparation method to extrude high silica-containing materials, specifically a class of zeolites belonging to the ZSM family. The method involves the use of an extrusion aid, which is added to the silica-rich material prior to extrusion. Basic salts and hydroxides of the Group I metals are broadly taught as acceptable extrusion aids. However, only one compound is actually named or exemplified, that being sodium hydroxide. After the catalyst is extruded, the extrusion aid is neutralized with an acidic solution and washed from the formed particle prior to the steps of drying and calcination. The washing is performed to avoid entrapment of the sodium cations in the composite. Neutralization with the acidic solution is conducted to reduce the basicity of the composite which is imparted to the composite by the sodium hydroxide. In contradistinction to this reference, the instant invention is directed at not only preserving basicity of an extruded catalyst composite but in addition, incorporating alkali metal cations into the final catalytic composite. Moreover, the '815 patent has not recognized the utility of potassium hydroxide in combination with a nonacidic potassium form type-L zeolite.